Hi, I was wondering if anyone could could help me out with my situation. I recently noticed that I have a firm mass behind my left testicle. It is pretty much painless, but I have concern that it may be cancerous. The unusual thing I have found is that when I sit down I can not feel it, but when I stand I can feel it. If anyone knows what the deal is with my condition it would be greatly, greatly appreciated if you could respond, I'm getting pretty nervous. Thanks.